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<tr class="t-nv"><td colspan="5"> <a href="attributes.html" title="cpp/language/attributes"> Attributes</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Types</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="typedef.html" title="cpp/language/typedef"> <code>typedef</code> declaration</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="type_alias.html" title="cpp/language/type alias"> Type alias declaration</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Casts</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="implicit_cast.html" title="cpp/language/implicit conversion"> Implicit conversions</a> - <a href="explicit_cast.html" title="cpp/language/explicit cast"> Explicit conversions</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="static_cast.html" title="cpp/language/static cast"><tt>static_cast</tt></a> - <a href="dynamic_cast.html" title="cpp/language/dynamic cast"><tt>dynamic_cast</tt></a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="const_cast.html" title="cpp/language/const cast"><tt>const_cast</tt></a> - <a href="reinterpret_cast.html" title="cpp/language/reinterpret cast"><tt>reinterpret_cast</tt></a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Memory allocation</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="new.html" title="cpp/language/new"> <code>new</code> expression</a></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="delete.html" title="cpp/language/delete"> <code>delete</code> expression</a></td></tr>
</table></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"> <a href="classes.html" title="cpp/language/classes"> Classes</a></td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <strong class="selflink"> Class declaration</strong></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="initializer_list.html" title="cpp/language/constructor"> Constructors</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="this.html" title="cpp/language/this"> <code>this</code> pointer</a></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="access.html" title="cpp/language/access"> Access specifiers</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="friend.html" title="cpp/language/friend"> <code>friend</code> specifier</a></td></tr>
</table></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Class-specific function properties</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="virtual.html" title="cpp/language/virtual"> Virtual function</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="override.html" title="cpp/language/override"> <code>override</code> specifier</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="final.html" title="cpp/language/final"> <code>final</code> specifier</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="explicit.html" title="cpp/language/explicit"><tt>explicit</tt></a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="static.html" title="cpp/language/static"><tt>static</tt></a> </td></tr>
</table></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Special member functions</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="default_constructor.html" title="cpp/language/default constructor"> Default constructor</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="copy_constructor.html" title="cpp/language/copy constructor"> Copy constructor</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="move_constructor.html" title="cpp/language/move constructor"> Move constructor</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="as_operator.html" title="cpp/language/as operator" class="mw-redirect"> Copy assignment</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="move_operator.html" title="cpp/language/move operator" class="mw-redirect"> Move assignment</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="destructor.html" title="cpp/language/destructor"> Destructor</a></td></tr>
</table></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"><a href="templates.html" title="cpp/language/templates"> Templates</a> </td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="class_template.html" title="cpp/language/class template"> Class template </a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="function_template.html" title="cpp/language/function template"> Function template </a></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="template_specialization.html" title="cpp/language/template specialization"> Template specialization</a></td></tr>   
<tr class="t-nv"><td colspan="5"> <a href="parameter_pack.html" title="cpp/language/parameter pack"> Parameter packs</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
</table></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"> Miscellaneous </td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="asm.html" title="cpp/language/asm"> Inline assembly</a></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="history.html" title="cpp/language/history"> History of C++</a></td></tr>
</table></div></td></tr>
</table></div>
</div><div></div></div></div></div><div class="t-navbar-sep"> </div><div class="t-navbar-head"><a href="classes.html" title="cpp/language/classes"> Classes</a><div class="t-navbar-menu"><div><div style="display:inline-block">
<div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv-h2"><td colspan="5"> General </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="classes.html" title="cpp/language/classes"> Overview</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <strong class="selflink"> <code>class</code>/<code>struct</code> types</strong> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="union.html" title="cpp/language/union"> <code>union</code> types</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="injected-class-name.html" title="cpp/language/injected-class-name"> Injected-class-name</a> </td></tr>
<tr class="t-nv-h2"><td colspan="5"> Members </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="data_members.html" title="cpp/language/data members">Data members</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="static.html" title="cpp/language/static">Static members</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="this.html" title="cpp/language/this">The <code>this</code> pointer</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="nested_classes.html" title="cpp/language/nested classes" class="mw-redirect">Nested classes</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="member_template.html" title="cpp/language/member template">Member templates</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="bitfield.html" title="cpp/language/bit field">Bit fields</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="using_declaration.html" title="cpp/language/using declaration"><code>using</code>-declarations</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="member_functions.html" title="cpp/language/member functions">Member functions</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="access.html" title="cpp/language/access">Member access specifiers</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="initializer_list.html" title="cpp/language/constructor">Constructors and member initializer lists</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="data_members.html#Member_initialization" title="cpp/language/data members">Default member initializer</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="friend.html" title="cpp/language/friend"><code>friend</code> specifier</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="explicit.html" title="cpp/language/explicit"><code>explicit</code> specifier</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="converting_constructor.html" title="cpp/language/converting constructor">Converting constructor</a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Special member functions </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="default_constructor.html" title="cpp/language/default constructor">Default constructor</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="copy_constructor.html" title="cpp/language/copy constructor">Copy constructor</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="move_constructor.html" title="cpp/language/move constructor">Move constructor</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="as_operator.html" title="cpp/language/copy assignment">Copy assignment operator</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="move_operator.html" title="cpp/language/move assignment">Move assignment operator</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="destructor.html" title="cpp/language/destructor">Destructor</a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Inheritance </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="derived_class.html" title="cpp/language/derived class">Base and derived classes</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="ebo.html" title="cpp/language/ebo">Empty base optimization (EBO)</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="virtual.html" title="cpp/language/virtual">Virtual member functions</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="abstract_class.html" title="cpp/language/abstract class">Pure virtual functions and abstract classes</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="override.html" title="cpp/language/override"><tt>override</tt></a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="final.html" title="cpp/language/final"><tt>final</tt></a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
</table></div>
</div><div></div></div></div></div><div class="t-navbar-sep"> </div></div>
<p>Classes are user-defined types, defined by class-specifier, which appears in <span class="t-spar">decl-specifier-seq</span> of the <a href="declarations.html" title="cpp/language/declarations">declaration</a> syntax. The class specifier has the following syntax:
</p>
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">class-key</span> <span class="t-spar">attr</span><span class="t-mark">(optional)</span> <span class="t-spar">class-head-name</span> <code><b>final</b></code><span class="t-mark">(optional)</span> <span class="t-spar">base-clause</span><span class="t-mark">(optional)</span> <code><b>{</b></code> <span class="t-spar">member-specification</span> <code><b>}</b></code>
</td>
<td> (1)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">class-key</span> <span class="t-spar">attr</span><span class="t-mark">(optional)</span> <span class="t-spar">base-clause</span><span class="t-mark">(optional)</span> <code><b>{</b></code> <span class="t-spar">member-specification</span> <code><b>}</b></code>
</td>
<td> (2)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<div class="t-li1"><span class="t-li">1)</span> named class definition</div>
<div class="t-li1"><span class="t-li">2)</span> unnamed class definition</div>
<table class="t-par-begin">


<tr class="t-par">
<td> <span class="t-spar">class-key</span>
</td>
<td> -
</td>
<td> one of <a href="../keyword/class.html" title="cpp/keyword/class"><tt>class</tt></a>, <a href="../keyword/struct.html" title="cpp/keyword/struct"><tt>struct</tt></a> and <a href="../keyword/union.html" title="cpp/keyword/union"><tt>union</tt></a>. The keywords <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">class</span></span></span> and <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">struct</span></span></span> are identical except for the default <a href="access.html" title="cpp/language/access">member access</a> and the default <a href="derived_class.html" title="cpp/language/derived class">base class access</a>. If it is <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">union</span></span></span>, the declaration introduces a <a href="union.html" title="cpp/language/union">union type</a>.
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">attr</span>
</td>
<td> -
</td>
<td> <span class="t-mark-rev t-since-cxx11">(since C++11)</span> any number of <a href="attributes.html" title="cpp/language/attributes">attributes</a>, may include <a href="alignas.html" title="cpp/language/alignas"><code>alignas</code></a> specifier
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">class-head-name</span>
</td>
<td> -
</td>
<td> the name of the class that's being defined, optionally <a href="name.html#Qualified_identifiers" title="cpp/language/identifiers">qualified</a>
</td></tr>
<tr class="t-par">
<td> <code><b>final</b></code>
</td>
<td> -
</td>
<td> <span class="t-mark-rev t-since-cxx11">(since C++11)</span> if present, the class <a href="final.html" title="cpp/language/final">cannot be derived</a>
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">base-clause</span>
</td>
<td> -
</td>
<td> list of one or more base classes and the model of inheritance used for each (see <a href="derived_class.html" title="cpp/language/derived class">derived class</a>)
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">member-specification</span>
</td>
<td> -
</td>
<td> list of access specifiers, member object and member function declarations and definitions (<a href="class.html#Member_specification">see below</a>).
</td></tr></table>
<h3><span class="mw-headline" id="Forward_declaration">Forward declaration</span></h3>
<p>A declaration of the following form
</p>
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td class="t-sdsc-nopad"> <span class="t-spar">class-key</span> <span class="t-spar">attr</span> <span class="t-spar">identifier</span> <code><b>;</b></code>
</td>
<td class="t-sdsc-nopad">
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<p>Declares a class type which will be defined later in this scope. Until the definition appears, this class name has <a href="incomplete_type.html" title="cpp/language/incomplete type" class="mw-redirect">incomplete type</a>. This allows classes that refer to each other:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">class</span> Vector<span class="sy4">;</span> <span class="co1">// forward declaration</span>
 
<span class="kw1">class</span> Matrix
<span class="br0">{</span>
    <span class="co1">// ...</span>
    <span class="kw1">friend</span> Vector operator<span class="sy2">*</span><span class="br0">(</span><span class="kw4">const</span> Matrix<span class="sy3">&amp;</span>, <span class="kw4">const</span> Vector<span class="sy3">&amp;</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">class</span> Vector
<span class="br0">{</span>
    <span class="co1">// ...</span>
    <span class="kw1">friend</span> Vector operator<span class="sy2">*</span><span class="br0">(</span><span class="kw4">const</span> Matrix<span class="sy3">&amp;</span>, <span class="kw4">const</span> Vector<span class="sy3">&amp;</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
<p>and if a particular source file only uses pointers and references to the class, this makes it possible to reduce #include dependencies:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co1">// in MyStruct.h </span>
<span class="co2">#include &lt;iosfwd&gt; // contains forward declaration of std::ostream</span>
 
<span class="kw1">struct</span> MyStruct
<span class="br0">{</span>
    <span class="kw4">int</span> value<span class="sy4">;</span>
    <span class="kw1">friend</span> <a href="../io/basic_ostream.html"><span class="kw1733">std::<span class="me2">ostream</span></span></a><span class="sy3">&amp;</span> operator<span class="sy1">&lt;&lt;</span><span class="br0">(</span><a href="../io/basic_ostream.html"><span class="kw1733">std::<span class="me2">ostream</span></span></a><span class="sy3">&amp;</span> os, <span class="kw4">const</span> S<span class="sy3">&amp;</span> s<span class="br0">)</span><span class="sy4">;</span>
    <span class="co1">// definition provided in MyStruct.cpp file which uses #include &lt;ostream&gt;</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
<p>If forward declaration appears in local scope, it <i>hides</i> previously declared class, variable, function, and all other declarations of the same name that may appear in enclosing scopes:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> s <span class="br0">{</span> <span class="kw4">int</span> a<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> s<span class="sy4">;</span> <span class="co1">// does nothing (s already defined in this scope)</span>
 
<span class="kw4">void</span> g<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <span class="kw1">struct</span> s<span class="sy4">;</span> <span class="co1">// forward declaration of a new, local struct "s"</span>
              <span class="co1">// this hides global struct s until the end of this block</span>
 
    s<span class="sy2">*</span> p<span class="sy4">;</span>     <span class="co1">// pointer to local struct s</span>
 
    <span class="kw1">struct</span> s <span class="br0">{</span> <span class="kw4">char</span><span class="sy2">*</span> p<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// definitions of the local struct s</span>
<span class="br0">}</span></pre></div></div>
<p>Note that a new class name may also be introduced by an <a href="elaborated_type_specifier.html" title="cpp/language/elaborated type specifier">elaborated type specifier</a> which appears as part of another declaration, but only if <a href="lookup.html" title="cpp/language/lookup">name lookup</a> can't find a previously declared class with the same name.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">class</span> U<span class="sy4">;</span>
 
<span class="kw1">namespace</span> ns
<span class="br0">{</span>
    <span class="kw1">class</span> Y f<span class="br0">(</span><span class="kw1">class</span> T p<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// declares function ns::f and declares ns::T and ns::Y</span>
 
    <span class="kw1">class</span> U f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>          <span class="co1">// U refers to ::U</span>
 
    <span class="co1">// can use pointers and references to T and Y</span>
    Y<span class="sy2">*</span> p<span class="sy4">;</span>
    T<span class="sy2">*</span> q<span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<h3><span class="mw-headline" id="Member_specification">Member specification</span></h3>
<p>The member specification, or the <i>body</i> of a class definition, is a brace-enclosed sequence of any number of the following:
</p>
<div class="t-li1"><span class="t-li">1)</span> Member declarations of the form
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td class="t-sdsc-nopad"> <span class="t-spar">attr</span><span class="t-mark">(optional)</span> <span class="t-spar">decl-specifier-seq</span><span class="t-mark">(optional)</span> <span class="t-spar">member-declarator-list</span><span class="t-mark">(optional)</span> <code><b>;</b></code>
</td>
<td class="t-sdsc-nopad">
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<table class="t-par-begin">


<tr class="t-par">
<td> <span class="t-spar">attr</span>
</td>
<td> -
</td>
<td> <span class="t-mark-rev t-since-cxx11">(since C++11)</span> any number of <a href="attributes.html" title="cpp/language/attributes">attributes</a>
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">decl-specifier-seq</span>
</td>
<td> -
</td>
<td> sequence of <a href="declarations.html#Specifiers" title="cpp/language/declarations">specifiers</a>. It is only optional in the declarations of constructors, destructors, and user-defined type <a href="cast_operator.html" title="cpp/language/cast operator">conversion functions</a>
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">member-declarator-list</span>
</td>
<td> -
</td>
<td> similar to a <a href="declarations.html" title="cpp/language/declarations">init-declarator-list</a>, but additionally allows <a href="bitfield.html" title="cpp/language/bit field">bit-field declaration</a>, <a href="abstract_class.html" title="cpp/language/abstract class">pure-specifier</a><span class="t-rev-inl t-since-cxx11"><span>, and virt-specifier (<code><a href="override.html" title="cpp/language/override">override</a></code> or <code><a href="final.html" title="cpp/language/final">final</a></code>)</span> <span><span class="t-mark-rev t-since-cxx11">(since C++11)</span></span></span>, and does not allow <a href="direct_initialization.html" title="cpp/language/direct initialization">direct-non-list-initialization syntax</a>.
</td></tr></table>
<p>This declaration may declare <a href="static.html" title="cpp/language/static">static</a> and non-static <a href="data_members.html" title="cpp/language/data members">data members</a> and <a href="member_functions.html" title="cpp/language/member functions">member functions</a>, member <a href="typedef.html" title="cpp/language/typedef">typedefs</a>, member <a href="enum_class.html" title="cpp/language/enum">enumerations</a>, and <a href="nested_classes.html" title="cpp/language/nested classes" class="mw-redirect">nested classes</a>. It may also be a <a href="friend.html" title="cpp/language/friend">friend declaration</a>.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">class</span> S
<span class="br0">{</span>
    <span class="kw4">int</span> d1<span class="sy4">;</span>            <span class="co1">// non-static data member</span>
    <span class="kw4">int</span> a<span class="br0">[</span><span class="nu0">10</span><span class="br0">]</span> <span class="sy1">=</span> <span class="br0">{</span><span class="nu0">1</span>,<span class="nu0">2</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// non-static data member with initializer (C++11)</span>
 
    <span class="kw4">static</span> <span class="kw4">const</span> <span class="kw4">int</span> d2 <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// static data member with initializer</span>
 
    <span class="kw1">virtual</span> <span class="kw4">void</span> f1<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span> <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> <span class="co1">// pure virtual member function</span>
 
    <a href="../string/basic_string.html"><span class="kw1233">std::<span class="me2">string</span></span></a> d3, <span class="sy2">*</span>d4, f2<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// two data members and a member function</span>
 
    <span class="kw2">enum</span> <span class="br0">{</span>NORTH, SOUTH, EAST, WEST<span class="br0">}</span><span class="sy4">;</span>
 
    <span class="kw1">struct</span> NestedS
    <span class="br0">{</span>
        <a href="../string/basic_string.html"><span class="kw1233">std::<span class="me2">string</span></span></a> s<span class="sy4">;</span>
    <span class="br0">}</span> d5, <span class="sy2">*</span>d6<span class="sy4">;</span>
 
    <span class="kw1">typedef</span> NestedS value_type, <span class="sy2">*</span>pointer_type<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div></div>
<div class="t-li1"><span class="t-li">2)</span> Function definitions, which both declare and define <a href="member_functions.html" title="cpp/language/member functions">member functions</a> or <a href="friend.html" title="cpp/language/friend">friend functions</a>. A semicolon after a member function definition is optional. All functions that are defined inside a class body are automaticaly <a href="inline.html" title="cpp/language/inline">inline</a><span class="t-rev-inl t-since-cxx20"><span>, unless they are attached to a <a href="modules.html" title="cpp/language/modules">named module</a></span> <span><span class="t-mark-rev t-since-cxx20">(since C++20)</span></span></span>.
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">class</span> M
<span class="br0">{</span>
    <a href="../types/size_t.html"><span class="kw106">std::<span class="me2">size_t</span></span></a> C<span class="sy4">;</span>
    <a href="../container/vector.html"><span class="kw1272">std::<span class="me2">vector</span></span></a><span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span> data<span class="sy4">;</span>
<span class="kw1">public</span><span class="sy4">:</span>
    M<span class="br0">(</span><a href="../types/size_t.html"><span class="kw106">std::<span class="me2">size_t</span></span></a> R, <a href="../types/size_t.html"><span class="kw106">std::<span class="me2">size_t</span></span></a> C<span class="br0">)</span> <span class="sy4">:</span> C<span class="br0">(</span>C<span class="br0">)</span>, data<span class="br0">(</span>R<span class="sy2">*</span>C<span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span> <span class="co1">// constructor definition</span>
 
    <span class="kw4">int</span> operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span>size_t r, size_t c<span class="br0">)</span> <span class="kw4">const</span> <span class="co1">// member function definition</span>
    <span class="br0">{</span>
        <span class="kw1">return</span> data<span class="br0">[</span>r <span class="sy2">*</span> C <span class="sy2">+</span> c<span class="br0">]</span><span class="sy4">;</span>
    <span class="br0">}</span>
 
    <span class="kw4">int</span><span class="sy3">&amp;</span> operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span>size_t r, size_t c<span class="br0">)</span>      <span class="co1">// another member function definition</span>
    <span class="br0">{</span>  
        <span class="kw1">return</span> data<span class="br0">[</span>r <span class="sy2">*</span> C <span class="sy2">+</span> c<span class="br0">]</span><span class="sy4">;</span>
    <span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div></div>
<div class="t-li1"><span class="t-li">3)</span> <a href="access.html" title="cpp/language/access">Access specifiers</a> <code>public:</code>, <code>protected:</code>, and <code>private:</code>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">class</span> S
<span class="br0">{</span>
<span class="kw1">public</span><span class="sy4">:</span>
    S<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>          <span class="co1">// public constructor</span>
    S<span class="br0">(</span><span class="kw4">const</span> S<span class="sy3">&amp;</span><span class="br0">)</span><span class="sy4">;</span>  <span class="co1">// public copy constructor</span>
    <span class="kw1">virtual</span> ~S<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// public virtual destructor</span>
<span class="kw1">private</span><span class="sy4">:</span>
    <span class="kw4">int</span><span class="sy2">*</span> ptr<span class="sy4">;</span> <span class="co1">// private data member</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div></div>
<div class="t-li1"><span class="t-li">4)</span> <a href="using_declaration.html" title="cpp/language/using declaration">Using-declarations</a>:
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">class</span> Base
<span class="br0">{</span>
<span class="kw1">protected</span><span class="sy4">:</span>
    <span class="kw4">int</span> d<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">class</span> Derived <span class="sy4">:</span> <span class="kw1">public</span> Base
<span class="br0">{</span>
<span class="kw1">public</span><span class="sy4">:</span>
    <span class="kw1">using</span> Base<span class="sy4">::</span><span class="me2">d</span><span class="sy4">;</span> <span class="co1">// make Base's protected member d a public member of Derived</span>
    <span class="kw1">using</span> Base<span class="sy4">::</span><span class="me2">Base</span><span class="sy4">;</span> <span class="co1">// inherit all bases' constructors (C++11)</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div></div>
<div class="t-li1"><span class="t-li">5)</span> <a href="static_assert.html" title="cpp/language/static assert"><code>static_assert</code></a> declarations:
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> T<span class="sy1">&gt;</span>
<span class="kw1">struct</span> Foo
<span class="br0">{</span>
    static_assert<span class="br0">(</span><a href="../types/is_floating_point.html"><span class="kw464">std::<span class="me2">is_floating_point</span></span></a><span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">value</span>, <span class="st0">"Foo&lt;T&gt;: T must be floating point"</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div></div>
<div class="t-li1"><span class="t-li">6)</span> <a href="member_template.html" title="cpp/language/member template">member template declarations</a>:
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> S
<span class="br0">{</span>
    <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> T<span class="sy1">&gt;</span>
    <span class="kw4">void</span> f<span class="br0">(</span>T<span class="sy3">&amp;&amp;</span> n<span class="br0">)</span><span class="sy4">;</span>
 
    <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> CharT<span class="sy1">&gt;</span>
    <span class="kw1">struct</span> NestedS
    <span class="br0">{</span>
        <a href="../string/basic_string.html"><span class="kw1231">std::<span class="me2">basic_string</span></span></a><span class="sy1">&lt;</span>CharT<span class="sy1">&gt;</span> s<span class="sy4">;</span>
    <span class="br0">}</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div></div>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<div class="t-li1"><span class="t-li">7)</span> <a href="type_alias.html" title="cpp/language/type alias">alias declarations</a>:
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> T<span class="sy1">&gt;</span>
<span class="kw1">struct</span> identity
<span class="br0">{</span>
    <span class="kw1">using</span> type <span class="sy1">=</span> T<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div></div>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx17"><td>
<div class="t-li1"><span class="t-li">8)</span> <a href="deduction_guide.html" title="cpp/language/deduction guide" class="mw-redirect">deduction guides</a> of member class templates:
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> S
<span class="br0">{</span>
    <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> CharT<span class="sy1">&gt;</span>
    <span class="kw1">struct</span> NestedS
    <span class="br0">{</span>
        <a href="../string/basic_string.html"><span class="kw1231">std::<span class="me2">basic_string</span></span></a><span class="sy1">&lt;</span>CharT<span class="sy1">&gt;</span> s<span class="sy4">;</span>
    <span class="br0">}</span><span class="sy4">;</span>
 
    <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> CharT<span class="sy1">&gt;</span>
    NestedS<span class="br0">(</span><a href="../string/basic_string.html"><span class="kw1231">std::<span class="me2">basic_string</span></span></a><span class="sy1">&lt;</span>CharT<span class="sy1">&gt;</span><span class="br0">)</span> <span class="sy2">-</span><span class="sy1">&gt;</span> NestedS<span class="sy1">&lt;</span>CharT<span class="sy1">&gt;</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div></div>
</td>
<td><span class="t-mark-rev t-since-cxx17">(since C++17)</span></td></tr>
</table>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx20"><td>
<div class="t-li1"><span class="t-li">9)</span> <a href="enum_class.html#Using-enum-declaration" title="cpp/language/enum">Using-enum-declarations</a>:
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw2">enum</span> <span class="kw1">class</span> color <span class="br0">{</span> red, orange, yellow <span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> highlight
<span class="br0">{</span>
    <span class="kw1">using</span> <span class="kw2">enum</span> color<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div></div>
</td>
<td><span class="t-mark-rev t-since-cxx20">(since C++20)</span></td></tr>
</table>
<h3><span class="mw-headline" id="Local_classes">Local classes</span></h3>
<p>A class declaration can appear inside the body of a function, in which case it defines a <i>local class</i>. The name of such a class only exists within the function scope, and is not accessible outside.
</p>
<ul><li> A local class cannot have static data members
</li><li> Member functions of a local class have no linkage
</li><li> Member functions of a local class have to be defined entirely inside the class body
</li><li> Local classes <span class="t-rev-inl t-since-cxx14"><span>other than <a href="lambda.html" title="cpp/language/lambda">closure types</a></span> <span><span class="t-mark-rev t-since-cxx14">(since C++14)</span></span></span> cannot have member templates
</li><li> Local classes cannot have <a href="friend.html#Template_friends" title="cpp/language/friend">friend templates</a>
</li><li> Local classes cannot define <a href="friend.html" title="cpp/language/friend">friend functions</a> inside the class definition
</li><li> A local class inside a function (including member function) can access the same names that the enclosing function can access.
</li></ul>
 <table class="t-rev-begin">
<tr class="t-rev t-until-cxx11"><td>
<ul><li> local classes could not be used as template arguments
</li></ul>
</td>
<td><span class="t-mark-rev t-until-cxx11">(until C++11)</span></td></tr>
</table>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;vector&gt;</span>
<span class="co2">#include &lt;algorithm&gt;</span>
<span class="co2">#include &lt;iostream&gt;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <a href="../container/vector.html"><span class="kw1272">std::<span class="me2">vector</span></span></a><span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span> v<span class="br0">{</span><span class="nu0">1</span>, <span class="nu0">2</span>, <span class="nu0">3</span><span class="br0">}</span><span class="sy4">;</span>
 
    <span class="kw1">struct</span> Local
    <span class="br0">{</span>
        <span class="kw4">bool</span> operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span><span class="kw4">int</span> n, <span class="kw4">int</span> m<span class="br0">)</span>
        <span class="br0">{</span>
            <span class="kw1">return</span> n <span class="sy1">&gt;</span> m<span class="sy4">;</span>
        <span class="br0">}</span>
    <span class="br0">}</span><span class="sy4">;</span>
 
    <a href="../algorithm/sort.html"><span class="kw1622">std::<span class="me2">sort</span></span></a><span class="br0">(</span>v.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, v.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span>, Local<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// since C++11</span>
 
    <span class="kw1">for</span> <span class="br0">(</span><span class="kw4">int</span> n <span class="sy4">:</span> v<span class="br0">)</span>
        <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> n <span class="sy1">&lt;&lt;</span> <span class="st0">' '</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>Output:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">3 2 1</pre></div></div> 
</div>
<h3><span class="mw-headline" id="Defect_reports">Defect reports</span></h3>
<p>The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
</p>
<table class="dsctable" style="font-size:0.8em">
<tr>
<th> DR
</th>
<th> Applied to
</th>
<th> Behavior as published
</th>
<th> Correct behavior
</th></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/1693.html">CWG 1693</a>
</td>
<td> C++98
</td>
<td> member declarations could not be empty
</td>
<td> empty declaration allowed
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/1930.html">CWG 1930</a>
</td>
<td> C++98
</td>
<td> <span class="t-spar">member-declarator-list</span> could be empty when <span class="t-spar">decl-specifier-seq</span><br>contains a storage class specifier or cv qualifier
</td>
<td> the list must not be empty
</td></tr></table>
<h3><span class="mw-headline" id="See_also">See also</span></h3>
<table class="t-dsc-begin">

</table>

<!-- 
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